Thomas Borchert wrote:
Without knowing for sure, one of the possible factos for the Kennedy
accident is that he was not proficient enough with the autopilot to let
the machine fly anything but straight and level, e.g. the descent that
was initiated and started the accident sequence. The AP in that
aircraft would have been capable of doing a descent.

That's not what I get from reading the NTSB report. It says:

"The airplane was equipped with a Bendix/King 150 Series Automatic
Flight Control System (AFCS) [...]

The AFCS installed on the accident airplane had an altitude hold mode
that, when selected, allowed the airplane to maintain the altitude that
it had when the altitude hold was selected. The AFCS did not have the
option of allowing the pilot to preselect an altitude so that the
autopilot could fly to and maintain the preselected altitude as it
climbed or descended from another altitude."

Still, I would imagine you could leave it in "heading hold" mode,
disengage altitude hold, reduce power a bit, and the plane would enter a
perfectly controlled descent just based on trim. And I agree that a
polot properly trained in use of the autopilot should have known how to
do that.